On the other hand, generalised hydrodynamics (GHD) is concerned with the emergent, large-scale behaviours of integrable, quantum and classical, many-body systems out of equilibrium [1]}, [2]}, [3]}. Many-body systems in states that are inhomogeneous at large distance scales, and that evolve on long time scales, may be described using hydrodynamic principles, in terms of “fluid cells” and their emergent dynamics [4]}. Before the advent of GHD, however, a common paradigm was that the applicability of the hydrodynamic principles – including the key notion of local thermalisation – required the system to be chaotic. Integrable systems admit an infinite number of conserved quantities which restrict their dynamics, and are usually considered to be non-chaotic. Integrability profoundly affects non-equilibrium physics as the presence of infinitely many conservation laws impedes thermalisation, while the lack of macroscopic homogeneity makes implementing standard methods of many-body integrability based on the inverse scattering method, difficult.
